Seriously though, it’s probably worth investigating the fuel mix. Could be that jets are blocked, mixture is wrong, air filter is blocked etc. I’d start there.
Yep i agree with imperialdata, it does sound like a fuel problem a misfire (properly called afterfire) is an indication of incorrect mixture (usually weak) carbs need stripping & cleaning properly then setting up to stock spec & balancing, presuming the bike is standard on both intake & exhaust sides this should sort the problem.